If he continued to attack his belly, he would have a way to release Poseidon.
Zeus thought Jack was just attacking him. But when he saw Jack attacking the same place, he realized what was going on.
Jack wanted to attack his belly and take advantage of this opportunity to release Poseidon.
At the thought of this, he was very scared. He would never let Jack do such a thing again. No matter what, he would stop him.
Although Zeus wanted to stop Jack, he realized that he was in a very weak state now. There was no way for him to resist.
He realized how vulnerable he was now, but because of this, he felt more unconvinced. He would never admit defeat in front of Jack.
"It turns out that you have already thought of it."
When Jack was not noticing, Zeus took out his dagger and stabbed it into Jack's chest.
But Zeus was different from Jack. When Jack was with his wife, Artemis would immediately leave a mark on Jack's wound to treat Jack.